Most wheeled work machines include a differential assembly in the machine's power train which allows the separate members of each set of wheels to rotate at different speeds. This capacity for rotation at different speeds, known in the art as “differentiation,” is necessary to allow smooth turning without undue stress and wear on components of the power train, as well as the work machine's tires. When a work machine navigates a turn, wheels on the outside of the turn are typically rotated more rapidly than the wheels on the inside of the turn. A typical differential will allow at least some torque to continue to be applied to each wheel, while allowing the outer wheel to rotate more rapidly than in inner wheel during turning.
Work machine differentials may broadly be classified as “open” differentials, limited-slip differentials or locking differentials. A conventional open differential includes a ring gear configured to mesh with a drive gear, the drive gear in turn being coupled with a drive shaft of the work machine. Rotation of the ring gear via the drive gear in turn rotates a set of spider gears or pinions about a circular path. The pinions are in turn coupled with side gears connected to and rotating with first and second axle shafts of the work machine. Rotation of each axle shaft applies a torque to wheels of the work machine to propel the same. One function of the pinions is thus generally to transmit torque between the ring gear and the axle shafts, and ultimately to the wheels of the work machine. When the work machine is turning, for example, or one of the wheels encounters a slick spot in the work surface, rotation of the pinions about their mounting shafts provides a second function of allowing the axle shafts to rotate relative to one another. Despite counter-rotation of the axle shafts relative to one another, the ring gear can continue to rotate the entire set of pinions about their circular path such that the net rotation of the axle shafts is still in the same direction. In other words, while the respective axle shafts and side gears coupled therewith rotate in different directions relative to one another, the overall rotation of the axle continues to be in either a forward direction or a reverse direction, depending upon the selected transmission gear of the work machine. These general operating principles have long been known in the mechanical arts.
One known open differential design is shown in U.S. Pat. No. 6,361,467 to Chen, in particular a differential for use in an electrically powered vehicle. Chen includes a differential gear apparatus including a casing having a bearing surface for an axle. A parallel axis ring gear member is coupled with the casing, and configured to rotate a set of pinions mounted therein to in turn rotate axles of the vehicle and allow them to differentiate. The Chen configuration purportedly reduces noise and provides a steady transmission. While Chen may indeed achieve certain of its objectives, the design is limited in its robustness and overall structural integrity, making it suited to only certain applications. In particular, because the differential is often the “weak link” in a powertrain, there is often a premium on maximizing power and torque density that is not available with the Chen design.
In contrast to the design set forth in Chen, it is common in many modern differential assemblies to mount the pinions upon a member known in the art as a differential “spider.” A spider typically consists of a one-piece member having a plurality of arms which serve as support shafts for the pinions. The spider is then rotatably coupled with the ring gear. In this manner, rotation of the ring gear rotates the spider and in turn transmits torque to the side gears and connected axle shafts.
Many larger work machines, for example certain off highway trucks, operate in environments and under conditions where components of the powertrain, and in particular the differential assembly, can be subjected to extremely high loads. Such loads can consist of forces transmitted along an axis of the axle shafts coupled with the differential, as well as forces oriented transverse to the axle axes, and may even include significant rotational forces acting upon various parts of the differential. In an attempt to design differentials better able to react and withstand substantial loads without significant wear and/or failure, designs have arisen wherein the internal gears, including the pinions and side gears are mounted within a rotating, supporting housing. The housing, typically including multiple housing pieces is positioned about the spider and other components, and rotatably supported within an outer housing coupled to the work machine frame.
In one design common throughout the industry, the rotatable inner housing portions are coupled together at a bolted joint. The joint often serves the dual purposes of connecting the housing portions together via a torque transmitting joint, and capturing the spider within opposed partial bores in the housing. During assembly, the housing portions may be bolted together, and the differential spider press fit therebetween in a single assembly step. Thus, the joint lies in a plane intersecting the differential spider and intersecting the axes of rotation of the pinions mounted thereon. While this approach provides a relatively easy means of assembling the differential, the attempted dual purposes of the joint, i.e. joining the housing portions and constraining movement of the spider, present a series of challenges.
In particular, to successfully couple the housing portions together and also press fit the spider into its mounting bores, the respective components of the assembly must typically be machined to relatively high tolerances. This invariably requires undesirable extra time and effort in the manufacturing process. In addition, while the dual purposes of the joint would seem to provide for ease of assembly, there are often tradeoffs in terms of overall durability and wear resistance of the differential components. This appears to be due at least in part to the fact that it is difficult to maximize the strength of the joint coupling the housing portions together without sacrificing the support function that the housing portions serve with respect to the differential spider, and vice versa.
Relatively small movements of the joint components in designs similar to the above can have the tendency to cause relatively rapid wear such as fretting in the joint faces of the bolted housing portions, the spider retention bores and surfaces of the spider itself. In certain designs, torque is transferred to the differential housing, and hence to the axle portions, via a 90° angle of the gear interface between the pinion and ring gear. This configuration can introduce wear and stress issues different from those observed with parallel axis gear interfaces for transmitting torque. While incremental improvements have been made in some instances by increasing the bolt torque of the bolted joint between the housing portions, failure and excessive wear remains a problem. In some cases, the wear can be severe enough to shorten the working life of the differential assembly, and prevent reuse of the differential components. Other challenges relating to less than optimal load capacity of portions of the differential assembly can include cracking of the rotating differential housing, retention failure of thrust plates for the side gears, thread tearing on the spider when spider retention nuts are removed, and abnormal or rapid wear on the differential gears themselves.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.